Definition of R-value in English:
The capacity of an insulating material to resist heat flow. The higher the R-value, the greater the insulating power.
- So, where R-values rate how much heat loss the material resists from passing through it, U-values rate how much heat the component allows to pass through it.
- Manufacturers usually represent the energy efficiency of windows in terms of their U-values (conductance of heat) or their R-values (resistance to heat flow).
- Don't forget that R-values are determined by material type, thickness, and installed weight per square foot, not by thickness alone.